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P.D. Hamilton, V. Seshadri, B. Rapp, N. Ravi; Biomechanics of the Porcine Lens . Invest. Ophthalmol. Vis. Sci. 2003;44(13):242.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To investigate presbyopia and clearly understand the dynamics of accommodation, the viscoelasticity of the lens, being the most compliant of the tissues involved, needs to be quantitatively characterized. The primary goal of this study is to determine the time-dependent mechanical properties of the intact lens and the lens substance, using the porcine lens as a model. Methods: Static Stress Scan Experiments were performed on both the intact lens and the lens substance using the Dynamic Mechanical Analyzer Model 7e (Perkin-Elmer, CT, USA). Digital video images of the lenses were taken, to calculate contact area and enable cross-sectional analysis. A static force scan from 0-25 mN was applied at a rate of 5 mN/min for all the lenses with capsules. Lenses without the capsule, were scanned from 0-10 mN at 1 mN/min. Values of Young's Modulus were obtained from the slope of stress vs. strain. Creep Test Creep-recovery experiments for the lenses with capsules were performed at a strain of 5%. The probe position was equilibrated for 3 minutes. The force was applied for two minutes and then three minutes was allowed for recovery. The lens substance was allowed to creep by its own weight for 10 minutes with no equilibration time. Experiments were set up in the humidifying chamber. The strain vs. time data was then analyzed using MATLAB 5.3. Results: The creep recovery data of the lens with the capsule was adequately fitted with a 2 GKM model. The Young's modulus of elasticity calculated from intact lens creep experiments at 6 mN (plus the weight of the lens) was 2950 ± 157 Pa. The Young's Modulus value obtained from creep data for the lens without capsule, allowing it to creep under its own weight, was 1119 ± 140 Pa. Conclusions: The visco-elastic characteristics of the whole lens as well as the lens substance were determined. A two-Voigt linear solid model described the creep recovery characteristic well. A membrane theory model is currently being developed to obtain the elastic properties of the capsule and the substance simultaneously. Such experimental lens data needs to be validated using materials of know moduli, within the capsular bag.
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